A 50-year-old woman was transported to a hospital complaining of marked general malaise and epigastralgia with diarrhea and vomiting. Her electrocardiogram showed sinus arrest with a duration of nearly 8 sec. Atrial pacing with an external pacemaker improved her symptoms promptly. Following transfer to our hospital 3 days later, bradyarrhythmia was not detected despite the removal of the external pacemaker. Transient atrial fibrillation was found in our hospital, and she was diagnosed as hyperthyroidism based on findings of finger tremor, exophthalmos, diffuse goiter and an abnormally high level of thyroid hormone. On cardiac catheterization, left ventriculography showed anterior wall hypokinesis and mild mitral regurgitation. Coronary arteriography showed the absence of organic stenosis. Right ventricular endomyocardial biopsy showed myocardial hypertrophy and partial disarray, but no findings of myocarditis. Electrophysiological study showed the normal upper range of AH-time (120 msec) and HV-time (50 msec), and prolongation of corrected sinus recovery time (CSRT, 955 msec). After a euthyroid state was successively induced for about 10 days by methylmercaptoimidazole therapy, AH-time, HV-time and CSRT were shortened to 85, 35 and 290 msec respectively. Her complaints and sick sinus syndrome disappeared after the treatment of hyperthyroidism without a pacemaker.